Method of making a zinc oxide phosphor



Nov. 14, i950 A. L. J. SMITH 2,529,711

METHOD 0F MAKING A ZINC OXIDE PHOSPHOR Filed May 27, 1948 7000 /gaaZmventor Ann-m3 L T. SMITH El! M i Gtorlleg Patented Nov.. 14, 1950UNITED lSTATES PATENT .OFFICE METHOD'OF MAKING A ZINC OXIDE PHOSPHORArthur L. J. Smith, Lancaster, Pa., assignor to f Radio-Corporation ofAmerica,` a corporation of Delaware Application May 27, 1948, SerialN0.`29,577

14 Claims.

. 1 This invention relates to lmaterialsv that fluoresce uponexcitation'by cathode rays, light and :ultra-violet waves, X-ray,andotherfforms of energy.

lIn someapplications 'of a fluorescent phosphor screen, itisgre'atlydesirable to utilize the ultraviolet emissioninstead of'thevisual emission of -the phosphor. Fory example, in va flying spotyscanning kinescope using a zinc'oxide phosphor screen, Tit has beenrecognized that the ultraviolet'outp'ut ofthe phosphor has a shorterdecay time than the visible component.. For such apfplications,` it isthereioredesirable toluse a zinc oxidephosphor having a high eiiiciencyin ultraviolet regions of the spectrum.

"An objectY of my inventionis to provide an improved crystalline`inorganic f phosphor having high efiiciency of luminescence intheultra-violet regions of then spectrum.

A further object-ofthe invention is to provide Yan improved ycrystallineinorganic phosphor having very short persistence of'decay.

Another object is vto'prepare a more elicient zinclioxide phosphor,lparticularly. for comparatively low vvoltage and high current densitycathode ray excitation.

A further objectof my invention'is to provide an easily controllablemethodtof preparing a zinc Y oxidev phosphor.

rAnother Vobject of myv invention is to provide Y a method of preparingzinc oxide phosphor whose characteristics are determined' byVcontrollable steps inthe method.

Other. objects4 will appear KYin the' following description,reference'being had to the drawing, which is a graphical representationof the'emissionfcharacteristics of three zincv oxide phosphors jpreparedaccording. to my invention. Y

. I have found that luminescent 'zinc oxide phosphorl Ahavingl goodv`characteristics can `rbe pre, pared by,4 firing the'zinc'oxide Ain areducingatla furnace. andtredt between 940 C. and 1000 C.

vfor 60' minutes-in ra reducingcarbon monoxide l atmosphere.

This Y1lringstep is followed by a second firing of thematerial in anitrogenat- -mosphere containing from 0.01% to 0.3% of oxygen YforVi'lve to twenty minutes at 'y 940 to 1000 C. The phosphor material isthen cooled in this mildly oxidizing atmosphere,

The f above-described procedure of producing zinc oxidelphosphoris oneywhich produces consistently -good'phosphor material having` highefciencyfin the ultraeviolet. The process is also zone which can beeasilycontrolled. The time of the first firing in CO isnot critical.Good phosvphor'mat'erial may be` produced for shorter `andlongerrperiods of time than the sixty minutes.

However,'the sixty'minute ring in CO results inthe most eilicientmaterial. A less efficient phosphor material'isfobtained'when thematerial `isriredin CO for more than or less than the sixty minuteoptimumf ringtime. f

AThe optimumring' time of sixty minutes reffduces the :f zinc voxide to:produce the ldesired amount fof zincmetal. kFiring for a longer time-crystalline ZnO` Vphospho-r 'which is undesirable. '.The secondfiringVin the nitrogen atmosphere tends to use yup the base material. The-temperaturerangeof'this' CO ring is such that rbelow 940 C. thereaction is too slow andV also the desiredv reaction will not take placebelow the vvboilingpoint. of zinc at 907 C. Above 1000 C.,

the ring'in CO produces a too rapidr growth of containingatrace ofoxygen provides a controlled reoxidation ofthe zinc produced in thefirst :ring step. *When the. zinc oxide is fired, as describedabove;fonten'minutes in themixed atmosphere ofV nitrogen-and oxygen, theresulting phosphor will have higher eciency-in the ultra-violet andlowerfefciencynin fithevisual region of the spectrum than'if Vthefr-ingin the mixed atmosphere were continuedffor twenty minutes. This fact isgraphically shown in the figure in which curve -was'fortwenty minutes.

i0"-repres'ents'the emission characteristics of a Yzincoxide phosphor,produced by the method described, lin which the second iiring wascontinued for ten minutesin the mixed atmosphere.

Curve l2"represents thefemission characteristics of-a zinc oxidevphosphor produced bythe same process butin which'the mixed atmospherefiring If the second firing is less thanlten minutes or'greater thantwenty minutes a less eilicient phosphor is obtained.

Also, the temperature of the second firing will "determine the'optimumring time, since `firing at a higher temperature requires ashorter optimum firing time and likewise a lower firing temperaturerequires a longer optimum ring time. A trace of oxygen in the coolingatmosphere tends to reduce the ultra-violet efficiency, but increasesthe efficiency of the visual emission.

The reactions taking place, during the preparation of the zinc oxidephosphor described above, are undoubtedly, first, a reduction of some ofthe zinc oxide to form free zinc metal within the batch, and secondly, adiffusion, volatilization, or reoxidation of the excess metal. It is notclear just what the function of this zinc metal is, yet it is thought tobe an activator. Zinc oxide phosphor has only been produced successfullyby ring the material in the presence of zinc metal. By ring the zincoxide material 60 minutes with a reducing CO atmosphere, zinc metal isformed in an amount in excess of that necessary to produce a vgoodphosphor. The excess amount of free zinc metal, produced in thereduction step, determines the time required to drive off or oxidizethis excess metal in the second stage of firing. It might be possible todetermine the actual time required for the reduction ring of zinc oxidefor any particular sized ,.1

batch in order to produce the exact amount of zinc metal required foractivation, however, the control of the firing time and ring atmospherewould be so extremely critical, that the process would be unmanageablefrom the standpoint of reproducibility. It is desirable that theparticle size of the zinc oxide material used be as small as possible toprevent the forming of a grainy screen, as during the firing of the zincoxide material, the particle size of the crystals tend to grow. This maybe partially controlled by not using higher firing temperatures thanthose given above. No attempt is made to mechanically reduce theparticle size of the phosphor formed, as such a step lessens itsefficiency.

Luminescent zinc oxide having good emission vin the visual region of thespectrum has been prepared in the past by firing the zinc oxide materialin hydrogen followed by cooling in air. In this process, zinc metal isproduced in the reduction ring step and any excess zinc metal iseliminated by reoxidation during the cooling in air. This process isdifcult to control, as the ring time must remain short and hence isextremely critical. The cooling condition is also extremely diiiicult tocontrol to produce a useful phosphor. The air, present during cooling,tends to reoxidize the zinc metal so rapidly that control of the processis lost. Methods of producing zinc oxide phosphors, where there is an f-an inert gas containing no oxygen. The phosphor material is then cooledin the mixed nitrogen-oxygen atmosphere or in the inert atmospherecontaining no oxygen. Although the preparation of zinc oxide phosphor bya hydrogen reduction firing is normally hard to control, the methodgiven here is not as critical, when the hydrogen firing is followed bythe firing in a controlled oxidizing medium. Cooling of the zinc oxidephosphor in air after a reduction ring, as previously practiced byothers, is dicult to control due to the too rapid reoxidation of thezinc metal.

In the controlled oxidation ring of zinc oxide material in mixednitrogen and oxygen, a greater proportion of oxygen will speed thereaction while a smaller proportion of oxygen will slow down thereaction. Also, the mixed nitrogen-oxygen firing may be followed by athird ring of the phosphor material at 940 C. to l000 C. in purenitrogen to increase the ultraviolet emission at the expense of thevisual emission. I have found, that after the rst reduction ring of thezinc oxide material, by appropriately varying the times of firing in themixed atmosphere of nitrogen and oxygen and of firing in pure nitrogen,I can produce a zinc oxide phosphor having a desired ratio ofultra-violet emission to Visual emission.

Another zinc oxide phosphor having high ultra-violet emission may beproduced by a first reduction firing of zinc oxide material in purecarbon monoxide for l0 to 60 minutes at a temperature between 940 C. to1000 C., or in hydrogen for '10 to l5 minutes at 970 C. to 1000D C. Thisis then followed by a second neutral ring of the material in a purenitrogen atmosphere for l0 to 60 minutes at 970 C to 1000 C.Alternatively, the range of ring temperature for this step may be 940 C.to 1000 C. The phosphor is then cooled in the pure nitrogen atmosphere.The time of the reduction firing is not critical and efficient phosphormaterial can be produced by shorter firing times. Again, the firstfiring determines the time required for the second ring. In this methodit is presumed that the excess zinc is removed by volatilization. Thesecond neutral ring step may be omitted if the phosphor material iscooled in a pure CO or pure N2 atmosphere immediately after the firstfiring in CO. However, the use of the second ring in N2 will producemore consistently phosphors of high eiciencies in the ultra-violet thanthose which use only a cooling step in CO or N2 after the firstreduction firing step because, as mentioned previously, the amount ofzinc cannot be controlled adequately by only a reduction firing. In thegure, curve I4 represents the emission characteristics of a zinc oxidephosphor produced by the method including the second step of ring inpure nitrogen. It can be seen that the phosphor represented by curve I4has essentially the same efiiciency in the ultra-violet as curve l0 buta lower visual efiiciency.

While I have indicated and described several systems for carrying myinvention into effect, it will be apparent to one skilled in the artthat my invention is by no means limite'd to the particularorganizations shown and described, but that many modifications may bemade without departing from the scope of my invention.

What I claim as new is:

1. The method of making a zinc oxide phosphor comprising the steps of,firing zinc oxide material between 940 C. and 1000o C. in a reducingatmosphere between 10 to 60 minutes, and ring the above red materialbetween 940 C. and 1000 C. in a nitrogen atmosphere containing theequivalent of from 0% to 0.3% of oxygen.

2. The method of making a zinc oxide phosphor comprising the steps of,firing zinc oxide material between 940 C. and '1000 C. in a reducingatmosphere between 10 to 60 minutes, ring the above fired materialbetween ,940 C. and 1000 C. in a nitrogen atmosphere containing theequivalent of from 0% to 0.3% of oxygen, and cooling the twice redphosphor in a nitrogen atmosphere containing the equivalent of from 0%to 0.3% of oxygen.

3. The method of making zinc oxide phosphor comprising the steps of,ring zinc oxide at 940 C. to 1000 C. in a reducing atmosphere, firingthe fired zinc oxide between 940 C. and 1000 C. in an atmosphere ofnitrogen containing 0.01% to 0.3% of oxygen, cooling the red zinc oxidein the atmosphere of nitrogen containing 0.01% to 0.3% of oxygen.

4. The method of making zinc oxide phosphor comprising the steps of,firing zinc oxide between 940 C. and 1000 C. in a reducing atmosphere ofcarbon monoxide for at least minutes, firing the red zinc oxide in anatmosphere of nitrogen containing 0.01% to 0.3% of oxygen at 940 C. to1000 C. for five to twenty minutes, and cooling in an atmosphere ofnitrogen containing 0.0'1% to 0.3% of oxygen.

5. The method of making zinc oxide phosphor comprising the steps of,firing zinc oxide between 940 C. and 1000 C. in a reducing atmosphere ofcarbon monoxide for at least ten minutes, firing the iired zinc oxide inan atmosphere of nitrogen containing 0.01% to 0.3% of oxygen at atemperature of 940 C. to 1000 C. for ve to ten minutes, and cooling thetwice fired zinc oxide in an atmosphere of nitrogen containing 0.01% to0.3% of oxygen.

6. The method of making zinc oxide phosphor comprising the steps of,firing zinc oxide in a reducing atmosphere of hydrogen at 940 C. to 1000C. for at least five minutes, ring the red zinc oxide in `an atmosphereof nitrogen containing 0.01% to 0.3% of oxygen at a temperature of 940C. to 1000" C. for at least ten minutes, and cooling the twice red zincoxide in an atmosphere of nitrogen containing 0.01% to 0.3% of oxygen.

7. The method of making a zinc oxide phosphor comprising the steps of,firing zinc oxide material in a reducing atmosphere, at a temperature of`at least 907 C. ring the above-fired material in an inert atmosphere ata temperature of at least 907 C., and cooling the fired material in aninert atmosphere.

8. The method of making a zinc oxide-phos phor comprising the steps of,firing zinc oxide in a reducing atmosphere between 940 C. and 1000 C.for at least 10 to 60 minutes, ring the above-said red zinc oxide in aninert atmosphere between 940 C. and '1000 C. for at least 10 to 60minutes, cooling the fired material in an inert atmosphere.

9. The method of making a zinc oxide phosphor comprising the steps of,ring zinc oxide in a reducing atmosphere between 940 C. and 1000 C.firing said red zinc oxide in an inert atmosphere of pure nitrogenbetween 940 C. and 1000 C. and cooling the red material in an inertatmosphere.

10. The method of making zinc oxide phosphor comprising thel steps of,firing zinc oxide in a reducing atmosphere of carbon monoxide between940 C. and 1000 C. for at least 10 to 60 minutes, firing the red zincoxide in a neutral atmosphere of pure nitrogen at 940 C. to 1000o C. forat least 10 to 60 minutes, and cooling the fired material in a neutralatmosphere of pure nitrogen.

11. The method of making zinc oxide phosphor comprising the steps of,ring zinc oxide in a reducing atmosphere of hydrogen at 940 C. to '1000C. firing the red zinc oxide in an atmosphere of pure nitrogen at940 C.to 1000 C., cooling the said iired material in an atmosphere of purenitrogen.

12. The method of making a zinc oxide phosphor comprising the steps of,ring zinc oxide material at a temperature of at least 907 C. in areducing atmosphere, firing the above red material at a temperature ofat least 907 C. in an atmosphere consisting essentially of a neutral gasand containing the equivalent of from 0% to 0.3% of oxygen, and coolingthe twice red phosphor in an atmosphere consisting essentially of aneutral gas and containing the equivalent of from 0% to 0.3% of oxygen.

13. The method of making a zinc oxide phosphor comprising the steps of,ring zinc oxide material between 940 C. and 1000 C. in a reducingatmosphere, ring the above red material at a temperature of 940 C. to1000 C. in a substantially neutral atmosphere selected from the groupconsisting of an inert atmosphere and an atmosphere consistingessentially of an inert gas containing the equivalent of from 0% to 0.3%oxygen, and cooling the twice fired phosphor in a substantially neutralatmosphere selected from the group consisting of an inert atmosphere andan atmosphere consisting essentially of an inert gas containing theequivalent of from 0% to 0.3% oxygen.

14. The method of making zinc oxide phosphor comprising the steps of,firing zinc oxide at a temperature of at least 907 C. in a reducingatmosphere, firing the red zinc oxide at a temperature of at least 907C. in a substantially neutral atmosphere selected from the groupconsisting of an inert atmosphere and an atmosphere consistingessentially of an inert gas containing the equivalent of from 0%,` to0.3% of oxygen, and cooling the twice red material also in an atmosphereselected from said group.

ARTHUR L. J. SMITH.

REFERENCES CITED The following references are of record in the le ofthis patent:

UNITED STATES PATENTS- Number Name Date 1,355,904 McKee Oct. 19, 19201,422,485 Lewis Jan. 16, 1923 1,838,359 Brinker Dec. 29, 1931 2,408,475Nickle Oct. 1, 1946

14. THE METHOD OF MAKING ZINC OXIDE PHOSPHOR COMPRISING THE STEPS OF,FIRING ZINC OXIDE AT A TEMPERATURE OF AT LEAST 90*C. IN A REDUCINGATMOSPHERE, FIRING THE FIRED ZINC OXIDE AT A TEMPERATURE OF AT LEAST907*C. IN A SUBSTANTIALLY NEUTRAL ATMOSPHERE SELECTED FROM THE GROUPCONSIST-